Control system for multiple-effect evaporators



April 6, 1965 G. HARMAN 3,175,755

CONTROL SYSTEM FOR MULTIPLE-EFFECT EVAPORATORS Filed Aug. 17, 1960 HEAVY BLACK LIQUOR STORAGE INVENTOR.

mu GEORGE L. HARMAN 3 3g ATTORNEY Levaporators in the United States Patent 3,176,755 C(BNTRUi SYESTEM FGR MULTIPLE-EFFECT EVAPURATORS George L. Harman, Milwaukee, Wis, assignor to Bailey Meter (Iompany, a corporation of Delaware Filed Aug. 17, 1960, Ser. No. 50,269 Claims. (Cl. 159-44} My invention relates to a control system for multipleeitect evaporators used in the. pulp industry for the concentration of thin black liquor to thick black liquor.

As well known, if the black liquor was discarded from the process after being washed. from the pulp it would be necessary to replace all of the chemicals charged to the digester and the cost of pulp would be prohibitive. Hence, economy dictates the recovery of the chemicals as completely as possible. The first step in the recovery process is the concentration of the thin, or Weak as it is sometimes called, black liquor to heavy black liquor suitable for further processing in a direct contact evaporator and recovery furnace.

Liquor received from the washers usually contains from 12 to solids consisting of spent chemicals and the non-cellulose constituents of the wood. After concentration in the evaporator system the solids content is ordinarily in the order of 45 to 55%. Essentially the concentration process in the evaporator system consists in boiling off a considerable portion of the water found inthe Weak liquor. While such boiling oil can be accomplished in a single evaporator economy dictates the use of several evaporators arranged in series 'or cascade as it may be called. With this arrangement live steam as the heating medium is admitted to the first evaporator in the series and the vapor generated in this evaporator then becomes the steam supply for the next evaporator and so on. The thin black liquor, having the lowest boiling point, is fed to the last evaporator in the series and passes in contra flow relation to the steam through the system. In such a multiple-effect arrangement one pound of steam admitted to the first evaporator willevaporate approximately one pound of water and the'pound of vapor so formed will evaporate approximately one pound of water in the second evaporator and so on down the line resultingin a material increase in steam economy over a single evaporator arrangement.

As evaporators or etfects are added a point is reached where a Working temperature difference between liquor and vapor ceases to exist and'the liquor in the evaporator inplace of boiling merely simmers. For this reason the effects or evaporators are usually limited to between 5 and 7. In contemporary practice saturated steam is admitted to the first effect at about 15 to p.s.i.g. and the vapor formedin the last effect discharges to a condenser operating at about to 27" of mercury vacuum; the

exact vacuum depending upon thetemperature and quantity of cooling water available.

While the multiple-effect evaporator system has materially increased the economy of 1 chemical recovery in the pulp industry it has presented a difiicult control problem because of the long time constants and small storage capacities involved. Without proper control, changes inliqluor feed flow, solids concentration, steam conditions, cleanliness of heat exchanger'surfaces and the like result in an output of black liquor of non-uniform concentration. Furthermore because of the longtime constants, an evaporator system once upset, may require hours of trial and error adjustment before it is again stabilized. My inventionis particularly directed to the solution of the control problems presented by the use of multiple-elfect pulp industry for the recovery of chemicals.

BJYMIES Patented Apr. 6, 1965 "ice In the drawing? FIG. 1 shows in diagrammatic form my invention applied to a typical multiple-eifect evaporator system.

Referring to FIG. 1, I show, for purposes of illustration, a. multiple-eliect system consisting of evaporators l-5 inclusive. Thin black liquor is fed to evaporators 4 and 5 in parallel from a storage tank 6 by means of a pump 7. I have shown thin liquor fed to evaporators 4 and Sin parallel as this represents a practice frequently used and the arrangement further serves to illustrate a feature of my invention as later explained in detail.

The effluent from evaporator 5 is discharged through a pipe 8 and forced into evaporator 3 through pipe 9 by pump it). The effluent from evaporator 4 is likewise fed to evaporator 3 through pipe 9 by means of a pump ll. The liquor after passing through evaporator 3 is then fed by a pump 12 to evaporator 2, thence fed by a pump 13 to evaporator l whence it is discharged through pipe 14 to a heavy black liquor storage tank 15.

Steam as a heating medium is supplied evaporator 1 through a pipe 16 from anysuitable source such as the extraction stage of a turbine, pressure reducing valve or the like (not shown) and preferably at constant pressure. As heretofore noted, in contemporary practice steam is supplied the system at about 15 to 20 p.s.i.gv saturated.

The condensate of the steam supplied evaporator l is discharged through a pipe 17. This condensate may be returned to a steam generator or wasted. As well known in the art the decision to return or waste is frequently automatically determined from a conductivity sensitive control, an increase in conductivity indicating contamination of the condensate by the liquor rendering it unfit for recycling through the steam generator.

Theheat of vaporization of the steam fed to evaporator l is transferred to the liquor passing therethrough causing the liquor to boil. The vapor so formed is discharged through a pipe 18 to evaporator 2 wherein it forms the heating medium for boiling the liquor passing therethrough. This process isrepeated in evaporators 3, 4 and 5, the vapor generated in evaporator 2 dis charging through a pipell9to evaporator 3, the vapor generated therein discharging through a pipe 20 to evaporator 4, the vapor generated therein discharging through a pipe :21 toevaporator 5. The vapor discharged from evaporatorS passes to a condenser 22 supplied with water through a pipe123. Specific mill conditions determine the type of condenser. used, thatis, whether it is a jet, or barometric as it is sometimes. called, or a surface type. i 1 i The condensate from the eiiects 2, 3, 4. and 5 is discharged through pipes 24, 25, 26 and .27 respectively. This condensate may be Wasted or reused depending upon mill conditions, due care being taken in regard to contamination by the liquor as outlined with respect to evaporator ll. In some systems the condensate from one effect is flashed to the next succeeding effect improving the economy of operation. I have purposely illustrated a simplified multiple-elfect evaporator systemto provide an adequate basis for the description of my invention to follow. As will be apparent to. those skilled intthe. art, ordinarily auxiliary devices and processes are incorporated in a multipleeifect evaporator system such as catchalls in the vapor outflow pipe from each evaporator which serve to refine the separation of vapor from liquor, condensate traps and the like. As such auxiliary devices and processes are not pertinent to an understanding of my invention such have not beenillustrated.

- The control embodying my invention as shown in FIG.

l is of the type commonly known as, pneumatically tem are well known in the art and'their operation readily V understood.

There is shown in the drawing a constant flow control of the thin liquor feed to the evaporator system with automatic ratioing of the feed among the evaporators, as thin liquor is fed to more than one evaporator. Flow transmitter 28 establishes a pneumatic loading pressure proportional to the total feed of the thin liquor to the system. .I have shown this flow transmitter and other transmitters used in my conrol schematically as such transmitters may be any one of several types available.

The loading pressure established by transmitter 28, proportional to the total feed of thin liquor to the system is introduced through pipe 28A into the B chamber of a relay 29 having proportional plus reset action. The

the actual rate of feed will be equal to the demand rate of feed. A convenient means of adjusting the set point loading pressure may be provided by a selector station such as' shown at 30 being of the type illustrated and described in United States Patent 2,747,595 issued to P. S.

Dickey on May 29, 1956.

The output pressure at D of relay 29 changes in proportion to changes in the difference between the loading pressures in chambers A and B and by virtue of the restricted connection to chamber C continues to change at a slowrate so long as the pressures in chambers A and B are unequal or depart from a predetermined relation. Thus the relay 29 is spoken of in the art as a proportional plus reset type in that it serves to cause immediate changes proportional to the amount of departure from a desired value and thereafter a continuing change until the actual value is restored to the desired value; i The output pressure at D of relay 29 after passing through selector station 30 is transmitted through a pipe 31 to a diaphragm operated valve 32 regulating the rate of thin-liquor feed to evaporator 5.

The loading-pressure established by the transmitter 28 is, as shown,'also transmitted through a pipe 288 to the chamber A of a relay 33 similar to the relay 29. Into the chamber B of this relay is introduced a loading pressure proportional to therate of flow of thin liquor to evaporator 4 generated by a flow transmitter 34. With respect to relay 33 the loading pressure generated by flow transmitter 28 may be considered as establishing the set point of the constant flow control regulating the liquor feed to evaporator 4, as the output pressure established at 'D of relay 33, transmitted through selector station 35,

is eitective for positioning a diaphragm operated valve 36. For any total rate of liquor feed, the ratio between the rates of teed to evaporators 4 and may be adjusted by means of a relay 36A and a manual loader 37. The rely 36A is in general similar to relay 29 but is provided with a bellows 38 for remote adjustment of the propori tional band as illustrated and described more particularly in United States Patent 2,743,710 issued to Jack F. Shannon on May 1, 1956. I have further shown the mechanism of a similar relay 49, to which reference will be made later, in diagrammatic form. More particularly the'ratio between the input pressure at A and output pressureat D of relay 36A is varied in accordance with the loading pressure in a bellows 38 established by manual loader 37. Thus it will be evident that a one to one ratio between the flows to evaporators 4 and 5 may be established or any other desired ratio established by ad ustmg the proportionality between the input and output pressures of relay 36A.

The selector stations and serve primarily as a A means of readily transferring the constant flow control I have described from Automatic to Remote Manual or tor station is provided with a hand adjustable knob which when the control is on Remote Manual establishes the loading pressure for positioning the final control element \VlllClldVllh respect to selector station 39 is the valve 32 and Wlih respect to the selector station 35 is the valve 36. In accordance with my invention the rate of steam flow to the first elfec-t is maintained in desired proportion to the total rate on. liquor feed to the system. To accomplish this I show in the drawing a constant flow control of the steam flow, the set point of which is adjusted from total liquor feed flow. There-in I show the loading pressure established by the transmitter 28 introduced by a p pe 28C into the A chamber of relay which as pre-' viously mentioned is provided with proportional plus reset action. Into the B chamber of this relay the load mg pressure generated by a flow transmitter 41, as modified by relay 49, is introduced. Thus by virtue of relay 40 the rate of steam flow is maintained in predetermined ratio with the rate of liquor feed. The steam flow con stant fiow control loop is shown as provided with a selec tor station 42 and a diaphragm operated flow control valve 43 as the final control element. Changes in feed will effect immediately and proportionate changes in steam flow thereby maintaining the heat input to the systern in proportion to liquor feed resulting in an outflow of liquor from the system of uniform concentration. Qhanges in steam requirements occasioned by changes in liquor input are made before such changes cause an upset in the system.

Further in accordance with my invention the constant steam flow control is trimmed or readjusted automatcally in accordance with changes in density or concentratlon of the black liquor outflow from the evaporator system. I show by way of example the concentration or density of the black liquor determined by comparing the boiling temperatureof the black liquor to the temperature of boiling water at the pressure of the black liquor. The dilferenc e between these two temperatures is a measure of density or concentration of the liquor. Referring to the drawing I therein show a condenser 44 supplied with steam from pipe 16 through an adjustable-restriction 45. The condenser is maintained at the same pressure as the black liquor by a vent pipe 46 connected to the vapor space in evaporator 1. The arrangement therefore provides a means for maintaining a water sample at saturation temperature at the same pressure as the black liquor in evaporator 1. V v

. The black liquor leaving evaporator 1 is at boiling temperature, hence a difierential temperature device such as shown schematically at 47 responsive to the temperature of the water in condenser 44 and to the temperature of the black liquor in pipe 14 maybe utilized to generate a loading pressure proportional to the density of the black liquor. This loading pressure after passing through a selector station 48 serves to adjust the proportional band of relay 49 disposed'in the loading line between flow transmitter 41 and relay '40. Changes in liquor density will thereby cause a change in the propor tion between the actual rate of steam flow and loading pressure admitted to chamber B of relay 40. "Thus for a given rate of thin liquor feed a new rate of steam flow will be maintained as required to hold a given density or concentration of the black liquor leaving the system. The

nand control as it is frequently called. Each selecselector station 48 provides a means for manually adjusting the ratio between liquor and steam inputs if desired.

As well known the stability of operation is improved by maintaining a predetermined absolute pressure in condenser 22, this in some cases may be accomplished by a single element control wherein an absolute pressure meter such as diagrammatically illustrated at 51 establishes a loading pressure proportional to the absolute pressure in the condenser 22 which either directly, or indirectly through a relay to give proportional plus reset action (not shown), controls the rate of flow of cooling water to the condenser by positioning a diaphragm valve 52.

Certain types of surfaces condensers possess undesirable time lag characteristics. Where such a condenser is used it is desirable to provide a two element control such as I have shown which regulates the flow oi cooling Water to maintain a predetermined temperature of the cooling water in the condenser. The predetermined temperature is adjusted as required to maintain the desired absolute pressure in the condenser. I have schematically shown such an arrangement in the drawing wherein a temperature transmitter 53 establishes a loading pressure proportional to the temperature of the cooling water in or leaving the condenser and which through a relay 54 provides an output pressure for positioning the valve 52. The set point of this constant temperature control is adjusted as required to maintain a predetermined absolute pressure in the condenser by introducing the loading pressure generated by transmitter 51 into the B chamber of relay 54, which as shown, is provided with proportional plus reset action.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a control system for a multiple-effect evaporator system for the concentration of thin black liquor to thick black liquor, comprising in combination; measuring means of the rate of flow of heating medium furnished the system, regulating means of the heating me dium furnished the system operated by said measuring means and having an adjustable set point, means measuring the rate of flow of thin black liquor to the system, means operated by said last named means adjusting the set point of said regulating means to maintain a constant ratio between the rates of flow of heating medium and thin black liquor, means determining the concentration of the thick black liquor and means operated by said last named means adjusting the ratio maintained between the rates of flow of heating medium and thin black liquor in a direction tending to maintain a constant concentration of the thick black liquor.

2. In a control system for a multiple-effect evaporator system for the concentration of thin black liquor to thick black liquor composed of several evaporators arranged in cascade by vapor piping in which the water vapor boiled oif the liquor in one evaporator acts as the heating medium in the following evaporator and arranged to admit steam as a heating medium to the first evaporator and to admit thin black liquor to the last evaporator whereby the black liquor and steam pass through the system in contra-flow relationship, the combination comprising;

means producing a first control signal proportional in magnitude to the rate of flow of steam to the first evaporator, means producing a second control signal proportional in magnitude to the concentration of thick black liquor discharged from the first evaporator, means under the joint control of said signals producing a third control signal proportional in magnitude to the ratio be tween the magnitudes of said first and second signals, means producing a fourth control signal proportional in magnitude to the rate of flow of thin black liquor to the last evaporator, relay means under the joint control of said third and fourth signals generating a fifth control signal proportional in magnitude to the difference in magnitudes of the third and fourth signals, and regulating means of the rate of flow of steam to the first evaporator under the control of said fifth control signal whereby the rate of flow of steam is maintained in proportion to the rate of flow of thin black liquor and said proportion is changed inversely with changes in the concentration of the thick black liquor.

3. The control system as claimed in claim 2 for a multiple-effect evaporator system wherein thin. black liquor is fed to more than one evaporator in parallel which includes means maintaining a predetermined ratio between the flows of thin black liquor to the evaporators.

4. The control system as claimed in claim 2 for a multiple-effect evaporator system wherein the water vapor boiled off the liquor in the last eifect is discharged to a condenser in which the vacuum is maintained by cooling water, which includes means regulating the cooling water flow to maintain a predetermined temperature in the condenser and means adjusting the predetermined tempera.- ture maintained in accordance with the absolute pressure in the condenser in a direction to maintain the absolute pressure at a predetermined value.

5. The control system as claimed in claim 2 wherein said relay means includes means generating a change in the fifth control signal proportional to the time integral of the magnitude of the difference between said third and fourth control signals.

References Cited by the Examiner UNITED STATES PATENTS 2,040,284 5/36 Tell 159*44 2,073,825 3/37 Book an. 159-44 2,135,512 11/38 Holven 159 -44 2,181,731 11/39 Hinckley 1. 159 20 2,519,608 8/50 Stiilson 159-44 2,743,710 5/56 Shannon.

2,747,595 5/56 Dickey.

2,800,955 7/57 Honige-tral 159-44X 2,805,678 9/57 Panich.

2,840,154 6/58 Lankenau 159-20 FOREIGN PATENTS 1,223,627 6/60 France.

OTHER REFERENCES A. C. Camp: Integrated Control System Improves Evaporator Performance. Publication Sugar, June 1950, p 26, 27.

NORMAN YUDKOFF, Primary Examiner.

GEGRGE D. MITCHELL, CHARLES OCONNELL,

BENJAMIN BENDETT, Examiners. 

1. IN A CONTROL SYSTEM FOR A MULTIPLE-EFFECT EVAPORATOR SYSTEM FOR THE CONCENTRATION OF THIN BLACK LIQUOR TO THICH BLACK LIQUOR, COMPRISING IN COMBINATION: MEASURING MEANS OF THE RATE OF FLOW OF HEATING MEDIUM FURNISHED THE SYSTEM, REGULATING MEANS OF THE HEATING MEDIUM FURNISHED THE SYSTEM OPERATED BY SAID MEASURING MEANS AND HAVING AN ADJUSTABLE SET POINT, MEANS MEASURING THE RATE OF FLOW OF THIN BLACK LIQUOR TO THE SYSTEM, MEANS OPERATED BY SAID LAST NAMED MEANS ADJUSTING THE SET POINT OF SAID REGULATING MEANS TO MAINTAIN A CONSTANT RATIO BETWEEN THE RATE OF FLOW OF HEATING MEDIUM AND THIN BLACK LIQUOR, MEANS DETERMINING THE CONCENTRATION OF THE THICK BLACK LIQUOR AND MEANS OPERATED BY SAID LAST NAMED MEANS AJUSTING THE RATIO MAINTAINED BETWEEN THE RATES OF FLOW OF HEATING MEDIUM AND THIN BLACK LIQUOR IN A DIRECTION TENDING TO MAINTAIN A CONSTANT CONCENTRATION OF THE THICK BLACK LIQUOR. 